For instance, in a data signal line driving circuit and a scanning signal line driving circuit of an image display apparatus, a shift register has been widely used to adjust timing when sampling each data signal from an image signal, and to generate a scanning signal applied to each scanning signal line.
Meanwhile, the power consumption of an electronic circuit increases proportionally to a frequency, a load capacity, and the square of a voltage. Thus, a driving voltage has been set lower to reduce power consumption in a circuit connected to an image display apparatus, for example, in a circuit for generating an image signal transmitted to the image display apparatus, or in the image display apparatus itself.
Regarding a circuit using a polycrystalline silicon thin film transistor to secure a large display area, for example, in a pixel, a data signal line driving circuit, and a scanning signal line driving circuit, a driving voltage is not sufficiently reduced because a difference in a threshold voltage sometimes reaches about several [V] between substrates or on a single substrate. However, in a circuit using a monocrystalline silicon transistor such as the circuit for generating an image signal, a driving voltage is set at a value such as 5 [V], 3.3 [V], or a smaller value in many cases. Hence, when applying a clock signal lower than a driving voltage of the shift resistor, the shift register is provided with a level shifter for raising a voltage of the clock signal.
To be specific, as shown in FIG. 39, when a clock signal CK having an amplitude of about 5 [V] is applied to a conventional shift resistor 101, a level shifter 103 increases a voltage of the clock signal CK to a driving voltage (15[V]) of the shift resistor 101. The clock signal CK whose voltage has been increased is then applied to flip flops F1 to Fn, and a shift resistor section 102 shifts a start signal SP in synchronization with the clock signal CK.
However, in the conventional shift register 101, the clock signal CK is level-shifted before being transmitted to the flip flops F1 to Fn. Therefore, the longer a distance between the ends of the flip flops F1 to Fn, the longer a distance for transmission, resulting in larger power consumption.
To be specific, the capacity of a signal line for transmission increases with a transmitting distance. Thus, the level shifter 103 requires a larger driving capability, thereby increasing power consumption. Further, as in the construction in which the polycrystalline silicon thin film transistor is used to form the driving circuit including the level shifter 103, when the driving capability of the level shifter 103 is not sufficient, it is necessary to provide a buffer 104 between the level shifter 103 and the flip flops F1 to Fn as indicated by a dotted line of the FIG. 39 to transmit a waveform without deformation. Consequently, larger power consumption is necessary.
In recent years, an image display apparatus with a larger display screen and a higher resolution has been demanded, so that more steps have been required for the shift resistor section 102. Therefore, there has been an increasing need for a shift register and an image display apparatus that can achieve small power consumption even in the case of a large distance between the ends of the flip flops F1 to Fn.